When a generally conical spray is directed upon a workpiece surface to be coated, part of the treatment fluid unavoidably bypasses the workpiece and drops to the ground or strikes a wall of a surrounding spray booth in the absence of special precautions. Since the progressive accumulation of the hardened coating material on a floor or wall surface is undesirable, it has been proposed to establish a water curtain in the path of a generally horizontal spray, beyond the workpiece to be treated, to intercept the liquid particles bypassing the workpiece in order to remove them from the booth. A drawback of this method, however, is the fact that the intercepted excess treatment agent is wasted; it also becomes necessary to decontaminate the effluent water before allowing it to be further utilized or to be drained into a sewer.